This map indicates the area known as the Twin Tiers. Counties in dark green are almost always considered part of the Southern Tier. Counties in dark red indicate the Northern Tier. Counties in blue are those in Pennsylvania that use the name "Northern Pennsylvania," and counties in light green are those that sometimes consider themselves Southern Tier counties. The light blue areas are associated with the Twin Tiers but are not part of the region proper.

Though the region has much in common, there are a few differences. Most notable is the choice of sports teams: the Southern Tier has a tendency to support Buffalo and, in the eastern part of the state, New York City teams, while Northern Tier areas are more likely to root for teams from Philadelphia or Pittsburgh.

McKean, Potter and (less often) Cameron Counties refer to themselves as part of the Twin Tiers but almost never consider themselves part of the Northern Tier, instead going by the name "Northern Pennsylvania." There is often significant ambiguity in regions: for instance, the western part of the region (McKean and Potter Counties) will often associate themselves with St. Marys, a city larger than any city in that area but in Elk County, just south of what is considered "Northern Tier" by any standard. This broader area is also known as the Pennsylvania Wilds.

The region was historically a disputed territory in the history of the United States prior to its founding. The Northern Tier was claimed by Pennsylvania Colony and Connecticut Colony, while the Southern Tier was claimed by Pennsylvania, Massachusetts Bay Colony and New York Colony. Various treaties and land sales eventually placed the Northern Tier in Pennsylvania's hands and the Southern Tier in New York's.

The region's name originated in 1968 as part of a marketing campaign in the Elmira Star-Gazette that utilized twins.[1]